ccanlint: ignore "unused" non-ccan dependencies.

We can't really tell if non-ccan dependencies are used, so don't try.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tools/ccanlint/tests/depends_accurate.c

@@ -98,7 +98,7 @@ static void check_depends_accurate(struct manifest *m,
 	}
 
 	for (i = 0; i < core_deps; i++) {
-		if (!used[i])
+		if (!used[i] && strstarts(deps[i], "ccan/"))
 			score_file_error(score, m->info_file, 0,
 					 "%s is an unused dependency",
 					 deps[i]);